'At the bell the group just went and I did not have the response that I normally do. I felt ready to race and being in the third heat was an advantage but I didn't feel myself. I felt unwell after the race, my stomach is not good at all.'

Last week Twell's coach, Mick Woods, questioned the choice of Portugal for the British team's pre-championship training camp, saying: 'The weather isn't good for endurance athletes.

Learning curve: The 20-year-old struggled to cope

'With temperatures 30 degrees plus, to try and train is very difficult. It would've been better for us to have stayed at home.'

And Twell, who celebrated her 20th birthday on Monday, added: 'Portugal wastough, I have to admit. I have not dealt with it before, I had never been warm weather training before. My steady training was fine but for some sessions there was no shelter at all.'

Earlier in the session, Usain Bolt showed no ill effects from his record-breaking heroics on Sunday evening as he cruised into the second round of the 200 metres.

Bolt broke his own 100m world record in Berlin at the weekend with a stunning run of 9.58 seconds, taking 0.11secs off the mark he set in the Olympic final in Beijing.

But the Jamaican superstar insisted he would put the partying on hold with the 200m heats and quarter-finals taking place today, and the 22-year-old looked suitably refreshed as he eased to victory in his heat in 20.70s.

Lightning Bolt: The Jamaican speed king breezed through the blocks, past Eddy De Lepine (L) and Canada's Sam Effah, to cruise to 200m heat win

Marlon Devonish, Britain's only representative after the withdrawals of Dwain Chambers and trials winner Toby Sandeman, also advanced after winning his heatin 20.92s.

British trio Robert Tobin, Michael Bingham and Martyn Rooney, meanwhile all advanced to the semi-finals of the 400m, although Rooney had to work hard to qualify third after a false start in his heat.